Non-woven, fibrous webs have long been used to form filters for removing particulates from a fluid stream, particularly from air. Fibrous webs for filtration are typically fabricated by agglomerating randomly oriented fibers into a non-woven web. For some kinds of fibers, such as glass fibers, a binder is applied to the fibers so that the binder bonds the fibers at their intersections in order to hold the web together. For example, glass fibers are drawn from a molten mass of glass through a series of side by side extrusion orifices and then wound around a large rotating, cylindrical, metal drum. Binder is applied to the fibers either at a binder application station located between the orifices and the drum or the binder is applied to the fibers, such as by spraying, after they are wound on the drum. A typical prior art binder mixture comprises 65% urea formaldehyde and 35% water. After a sufficiently thick layer of fibers is wound around the drum, the layer of fibers is removed from the drum. The layer of fibers is subsequently blown, agitated, pulled or otherwise mechanically disturbed to assist in spreading the fibers apart and reorienting them into random entanglement to form a fibrous web or sheet material. The entire web and its binder is then dried and heated so that the binder is cured to bond the fibers where they intersect and contact each other.
Filter designers continuously seek to improve the particulate removal efficiency of filter media. One way to improve the efficiency of a filter is by providing a denser medium with smaller pores. However, the problem with this manner of improving efficiency is that it also increases the flow resistance, and therefore the pressure drop across the filter medium.
Another prior art method for enhancing the particulate removal efficiency of a fibrous filter is to spray a tackifier agent onto the filter medium after the non-woven fibrous web has been formed and the binder on the filter has been cured. For example, others in the prior art have sprayed oil, chlorinated paraffin, polybutene, phosphate esters, or other tackifier agents onto a filter medium web to promote the adhesion of particles and therefore improve filtration. However, because filters are designed to intercept particles entrained in air, the sprayed tackifier is concentrated near the surface of the filter medium and essentially non-existent in the interior. It is impossible to penetrate such a spray into the filter medium except by the use of excessive quantities of tackifier and at very high pressure. Furthermore, attempting to utilize such large quantities and high pressure makes the resulting product messy and difficult to handle. Consequently, this process is used only for high performance, relatively expensive filters, such as glass fiber bag filters for use in railroad diesel engines. Others have similarly applied tackifier agents to the surface of a filter medium web utilizing roll coating, but this provides even worse surface concentration.
The high concentration of tackifier on the surface of filter media as a consequence of rolling or spraying results in the accumulation during use of a very high concentration of adsorbed particles on the entrance side of the filter medium, causing such filters to become clogged more readily, quickly resulting in an increased pressure drop across the filter, and therefore reducing the service time of the filter.
Still others have immersed fibrous webs in a tackifier in order to deposit the tackifier on the fibers throughout the fibrous web. For example, filter media have been immersed in a polybutene which is diluted with a solvent. However, this process requires the removal of the solvent through some form of drying and recovery of the solvents in order to avoid pollution problems. An additional problem with the immersion approach is that the immersed filters, unless large quantities of solvents are used, apply a coating to the fibers which is excessively thick and messy.
Consequently, it is an object and feature of the present invention to provide a filter with a tacky fiber coating which is homogeneously distributed throughout the fibrous web filter medium, and to do so at an application rate which is easily controlled, and in a manner which avoids the use of solvents.